Cross-polarized control channels for loading fiber optic transport systems

ABSTRACT

A method of loading a fiber optic transport system includes adding one or more control channels to an optical fiber having one or more channels, wherein each of the control channels comprises a pair of signals that are cross-polarized and each of the pair of signals is at a different frequency from one another; measuring optical power on the optical fiber; and adjusting optical power of the one or more control channels based on the measured optical power.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application for patent is a continuation of U.S. Pat.Application No. 14/635,565, entitled “TRANSIENT AND SWITCHING EVENTSTABILIZATION OF FIBER OPTIC TRANSPORT SYSTEMS, filed on Mar. 2, 2015(now U.S. Pat. No. 9,755,776 issuing on Sept. 5, 2017), which claims thebenefit of U.S. Pat. No. 8,971,705, entitled “TRANSIENT AND SWITCHINGEVENT STABILIZATION OF FIBER OPTIC TRANSPORT SYSTEMS,” issued on Mar. 3,2015, each is assigned to the assignee hereof, and each is expresslyincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to optical communications, and inparticular to a method and system for stabilizing transient events on afiber optic transport system.

BACKGROUND OF THE INVENTION

Today's fiber optic transport systems are evolving in many ways. Theyare achieving longer transmission distances between electricalregeneration points through the introduction of coherent detection anddigital signal processing (DSP) technologies. Today's systems haveevolved from point-to-point systems to optical mesh deployments throughthe introduction of reconfigurable optical add drop multiplexer (ROADM)technologies. These changes were first applied to terrestrialtransmission systems. Recently, these technologies have been introducedinto submarine systems which have traditionally required specifictransponder technologies because of the long distances betweenelectrical regeneration points.

These changes present opportunities and challenges. The opportunity isto use the flexibility of the ROADM and the DSP to allow the path of thesignal to be switched in the optical domain. The challenge is tomaintain post forward error correction (FEC) error-free performance forall other signals in the transmission system while making these changes.

A photonic line system is a concatenation of optical amplifiers. Forexample, these may be a combination of Raman Amplifiers and Erbium DopedFiber Amplifiers (EDFAs). The performance of channels transmittedthrough the line system is optimal when the powers of the channels arecontrolled to balance noise and non-linear effects in the fiber. Theeffect of noise added by the optical amplifiers is minimized byincreasing the power of the channels. However, deleterious non-lineareffects are stronger at higher optical powers. Therefore, there is anoptimal power level for any individual channel. To complicate thisfurther, there may be an arbitrary number of these channels present in adense wavelength division multiplexing (DWDM) transmission system. Inaddition, the gain shape and noise performance across wavelength of theamplifiers is affected by the spectral loading of the system, i.e., thepopulation of other channels, the wavelengths they occupy, and theiroptical powers. Since channels in an active system may be added ordropped for a variety of reasons, the gain shape and noise performancechanges in an unpredictable manner. Non-linear effects are also affectedby the spectral loading in the fiber through interactions between thesechannels, for example, cross-phase modulation (XPM) which is dependenton the relative powers and difference in wavelength between interferingchannels.

While the addition or deletion of channels from a line system can besimulated, simulation involves a calculation which is time-consuming andcostly to perform within a network element. The simulation is alsosensitive to unknown parameters which are difficult to obtain, such asthe connector losses which may exist before the input or at the outputof the transmission fiber. It is difficult to differentiate this lossfrom the loss of the fiber itself, and these differences will change thesolution which is obtained through simulation. Therefore, in the absenceof simulation, the system cannot predict the final state of the channelswhich will result from any change.

Alternatively, the optimization of a line system can be achieved using acontrol system which measures a combination of total powers andper-channel powers at various points. Such a controller can beimplemented which can iterate to a solution which was not easilycalculated by the system, and such a solution will intrinsically includethe effect of unknown parameters described in the previous paragraph. Asecond advantage of this approach is that it will converge even if thecalculation of the solution is unknown or inaccurate.

A feedback control loop, e.g., a proportional integral derivative (PID)controller, can be used to optimize the set of channels which may bepresent at a given time. A typical PID controller may use the opticalsignal to noise ratio (OSNR) as a metric to optimize the channels whileimplementing an upper power limit for the channels which is known tolimit the non-linear effects to a manageable level.

A problem with this approach is its sensitivity to perturbations whichare a natural consequence of channel additions and deletions. Becausethe line system is essentially non-linear (a perturbation in onechannel's power affects other channels), the controller must be limitedto small changes to remain stable and converge. Since the controller isiterative, each of these changes must be allowed to convergeappropriately before applying another change. The result is that therequest to add or delete channels takes time. Although this time can bereduced, there will still be delays when reconfiguring services.

A second problem with this approach is its sensitivity to perturbationswhich are not intended but can be the result of failures of some part ofthe system. There are many such events which can happen, such asoperator error causing a fiber disconnect, equipment failures causingchanges in optical power, fiber cuts where a portion of the transmissionfiber is damaged or broken, etc. When these events occur there can be alarge change in the number of channels which are transmitted through aparticular fiber because of channel adds and drops through ROADM nodes.Although the control system can accommodate this type of change, it willtake time to converge to a new optimal condition. The performance of theremaining channels can be compromised during the time it takes for thisaction to complete.

Other solutions exist which try to keep the spectral power in the EDFAsrepresentative of the full fill spectrum under all conditions. Theseapproaches have worked in the past mainly because of the exclusive useof direct detection, and non-polarization multiplexed signals which werethe only optical receiving means available before the commercialintroduction of coherent optical receivers. In the presence of thesecoherent signals, which have been widely adopted in product and bystandards bodies, current solutions fail to provide the required systemlevel performance for reasons which will be described in the followingsections.

Therefore what is needed is a method and system for transient eventstabilization of fiber optic transport systems.

SUMMARY OF THE INVENTION

The present invention advantageously provides a method and system fortransient and switching stabilization of a fiber optic transport system.According to one aspect, the invention provides a method for powercontrol of signals carried by an optical fiber. The method includescoupling one or more data-bearing channels to the optical fiber, wherethe data-bearing channels are distributed among a plurality of frequencysub-bands. A set of control channels is also coupled to the opticalfiber. Each control channel includes a pair of signals at separatefrequencies. There is at least one control channel in each of theplurality of frequency sub-bands. The signals of a control channel arecross-polarized. The optical power in at least one of the plurality ofsub-bands is measured. Responsive to the measured optical power, theoptical power of a control channel is adjusted to maintain asubstantially constant power of a sub-band that contains the adjustedcontrol channel.

According to another aspect, the invention provides an apparatus forpower control of signals carried by an optical fiber. The apparatusincludes a plurality of lasers producing a plurality of controlchannels. Each control channel includes a pair of signals at separatefrequencies. Each control channel is in a separate one of a plurality offrequency sub-bands. A plurality of attenuators control attenuation of acorresponding control channel based on a plurality of correspondingfeedback attenuation control signals. A plurality of detectors detectpower in each of a plurality of corresponding optical signals receivedin the frequency sub-bands. A digital processor generates the pluralityof feedback attenuation control signals based on the detected power. Thefeedback attenuation control signals are provided to a corresponding oneof the plurality of attenuators.

According to yet another aspect, the invention provides an opticalcontrol channel signal generator. The optical control channel signalgenerator includes a plurality of lasers producing a plurality of signalpairs. Each signal pair forms a control channel in a correspondingseparate one of a plurality of sub-bands. The signals of each signalpair are separated by a first frequency interval. A combiner combines afirst signal and a second signal of a signal pair so that a polarizationof the first signal is orthogonal to a polarization of the secondsignal. The optical control channel signal generator also includes aplurality of attenuators, each attenuator attenuates a correspondingsignal pair based on a measured optical power of a correspondingsub-band.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, and theattendant advantages and features thereof, will be more readilyunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram of an exemplary system for transient andswitching stabilization of a fiber optic transport system constructed inaccordance with the principles of the present invention;

FIG. 2 is a diagram of an example of a spectrum containing controlchannels and separated into bins;

FIG. 3 is a diagram of an example showing the spacing of two controlchannels and dummy/data-bearing channels;

FIG. 4 is a block diagram of an exemplary channel controller forcontrolling the power of a plurality of control channels constructed inaccordance with the principles of the present invention; and

FIG. 5 is a flow chart of an exemplary process for adjusting power of acontrol channel.

DETAILED DESCRIPTION OF THE INVENTION

Before describing in detail exemplary embodiments that are in accordancewith the present invention, it is noted that the embodiments resideprimarily in combinations of apparatus components and processing stepsrelated to implementing a system and method for transient eventstabilization of fiber optic transport systems. Accordingly, the systemand method components have been represented where appropriate byconventional symbols in the drawings, showing only those specificdetails that are pertinent to understanding the embodiments of thepresent invention so as not to obscure the disclosure with details thatwill be readily apparent to those of ordinary skill in the art havingthe benefit of the description herein.

As used herein, relational terms, such as “first” and “second,” “top”and “bottom,” and the like, may be used solely to distinguish one entityor element from another entity or element without necessarily requiringor implying any physical or logical relationship or order between suchentities or elements.

One approach to stabilization of a fiber optic transport system toaccount for transient phenomena occurring with the addition or deletionof channels is to introduce ‘dummy’ channels. With the introduction ofdummy channels, the amplifier chain of the fiber optic transport systembehaves as if it has a full complement of channels. The power of thedummy channels may be specified at start up of the system. The dummychannels may be generated by using a broadband amplified spontaneousemission (ASE) source. The output of the ASE source may be filtered by awavelength selective switch (WSS). An alternative implementation mayemploy a wavelength-independent tap between the WSS and an outputamplifier and using a 1×1 WSS to comb and control the powers of thedummy channels.

Using an ASE source rather than using individual lasers offers severaladvantages. First, lasers tend to be of a single polarization, allowingfor accidental alignment through coupling or polarization modedispersion (PMD) in the transmission fiber and optical elements. Thisresults in an aggregate signal with a high degree of polarization (DOP).A high aggregate DOP is undesirable because it can induce penaltiesarising from polarization-dependent effects such as stimulated Ramanscattering (SRS) in the fiber. Another adverse consequence of high DOPis polarization hole burning that results in polarization-dependent gain(PDG) in the Erbium Doped Fiber Amplifiers (EDFAs).

Second, the inherent noise of the laser sources can interact with thesignal of the data-bearing channels, through non-linear interactionssuch as cross phase modulation (XPM), four wave mixing (FWM), cross gainmodulation (XGM), cross polarization modulation (XpolM). To reduce theseinteractions would require the use of lasers with a very low line width,which would increase the cost of introducing dummy channels.

The use of ASE dummy channels overcomes these limitations of usingindividual lasers. ASE is inherently depolarized, which yields a low DOPof the aggregate signal. ASE also exhibits a low spectral density which,when the total power in a channel pass band is the same as the power ofa data-bearing channel, drives a lower non-linear interaction than ifall channels were data-bearing. Therefore, ASE dummy channels aresuitable for pre-loading a fiber optic transport system. A constraint onthe use of ASE dummy channels, however, it that their power should becarefully controlled to ensure that the non-linear interactions with thedata-bearing channels be held to a manageable level.

With the use of ASE dummy channels, channel addition and deletions cannow be achieved through a substitution of the ASE channel for adata-bearing channel, or vice versa. The only perturbation the systemwill experience occurs during the switch time interval of the wavelengthselective switch (WSS), which can be on the order of 50 milliseconds(ms). This switching perturbation can be limited by limiting the numberof channels to be switched at the same time.

The introduction of dummy channels, therefore, solve some of theproblems associated with the intentional adding or deleting of channelsbut do not solve problems arising from failure conditions. In the eventof a failure, a plurality of data-bearing channels may be suddenlydropped faster than the WSS can switch to add dummy channels in theirplace. Also, per channel control of the WSS is usually achieved bytaking per channel measurements using an optical spectrum analyzer (OSP)or an optical channel monitor (OCM). These per channel measurements areunacceptably slow on the order of 100 ms for a full spectrummeasurement.

An alternative to adding dummy channels in the event of a failure is toincrease the power of the dummy channels to offset the drop in theoptical power since this change can be made relatively rapidly. However,this approach presents several challenges to be considered. First, thedummy channels whose power is to be increased might not exist if, forexample, the system is at full data-bearing channel capacity. Further,if the power of dummy channels is increased, so as to compensate forlost data-bearing channels, the noise of the ASE will be transferred tothe data bearing channels through non-linear interactions, which is notdesirable.

Instead of increasing the number or power of dummy channels in responseto a failure or other cause of reduction of data-bearing channels, aminimal number of control channels can be set aside. These controlchannels are spaced throughout the transmission band so as to adequatelyrepresent the full-fill condition from the point of view of an amplifierand fiber. In particular, large changes in dynamic gain tilt (DGT),spectral hole burning (SHB) in the EDFAs and stimulated Raman scattering(SRS) are avoided. The spectrum is subdivided into bins (frequencysub-bands). Each frequency sub-band has dummy channels, data-bearingchannels, and at least one control channel. The total power level of abin is set to a level that represents the full complement of channelswhich may be contained in the bin including the control channels. Assuch, during a fault where channels appear or disappear, the power ofthe control channels can be decreased or increased respectively.

Referring now to the drawing figures, in which like referencedesignators denote like elements, there is shown in FIG. 1 an exemplarysystem for transient and switching stabilization of a fiber optictransport system generally designated as “10.” A plurality ofdata-bearing channels on lines 12 are received by a wavelength selectiveswitch (WSS) 16. Also received by WSS 16 are a plurality of dummychannels on lines 14 generated by one or more amplitude spontaneousemission sources (ASE) 18. Although only one ASE generator is shown inFIG. 1, in some embodiments, a plurality of ASE generators may beprovided for reasons such as redundancy. The WSS 16 multiplexes thesignals from the lines 12 and 14 onto a single optical fiber 23. Theoptical signal carried by the fiber 23 may be input to an amplifier 20before being transmitted onto an optical transmission fiber 28. Notethat the terms fiber, optical fiber, and optical line are usedinterchangeably herein and refer to a fiber optic transmission line.

System 10 includes an optical line 22 that carries a control channelsignal from a processor 26. The processor 26 includes a plurality ofoptical sources 30 for the control channels. In one embodiment, there isat least one control channel signal for each of a plurality of frequencysub-bands. The optical sources may be low line width lasers with asingle polarization. A signal from any two of the optical sources 30 maybe combined by a polarization beam combiner (not shown) to produce anoptical control channel signal that is coupled to the line 22. Theoptical control channels on the optical line 22 are coupled to theoptical fiber 23.

The optical line 24 receives optical power from the optical fiber 23 viaa coupler 25. Optical line 24 carries a signal coupled from the fiber 23to the processor 26. The signal on the line 24 may be coupled to opticaldetectors 31 to detect optical power of each sub-band. The power of eachcontrol channel is adjusted by an optical power control unit 32 based onthe detected optical power of a corresponding frequency sub-band.

FIG. 2. is a diagram of an example of a spectrum containing controlchannels 32-40 and separated into bins (frequency sub-bands), e.g., bins1, 2, 3 and 4. Bin 2, for example, may have a total of 20 dummy anddata-bearing channels and two control channel signal pairs 36 spacedclosely together. During normal operation, all of the 20dummy/data-bearing channels are present at a nominal per-channel powerselected to achieve optimal transmission. The two control channels arealso assigned the nominal per-channel power level. A total target powerfor the bin is set to the total power of all these channels. When afault occurs, the power of the control channels is increased so that thetotal power of all channels of the bin is kept constant. When there ismore than a single control channel in a bin, weighting factors can beadjusted and applied to each control channel. For example, in FIG. 2,the first control channel signals in bin number 1 are weighted by 30%(2×15%) and the second control channel signals are weighted by 70%(2×35%),

Two low line width lasers with a low degree of polarization (DOP) arecoupled using a polarization beam combiner to create a control channel,such that the light from the two sources are orthogonal to each otherwhen coupled on to the fiber. Note that the polarization alignment ofeach control channel is independent of the polarization alignment of theother control channels.

The spacing of the pair of signals that make a control channel isselected to be just great enough to avoid a single polarization state ofthe control channel because a single polarization state would result ininstability. The spacing of the signal pair is also selected to be smallenough to reduce non-linear interactions between the signals of acontrol channel and the signals of the dummy/data-bearing channelsnearby the control channel.

FIG. 3 is an example of the spacing of the signals 42 and 44 making up acontrol channel. Dummy/data-bearing channels 46 are spaced apart atregular frequency intervals of 50 Giga-Hertz (GHz). The control channelsignal pair are offset from the regular spacing so that they areseparated by one half a frequency interval that separates thedummy/data-bearing channels. Thus, for the example of FIG. 3, thecontrol channel signal pairs are separated by 25 GHz. FIG. 3 also showsseveral four wave mixing (FWM) tones 48 that fall on the boundariesbetween channels, thereby minimizing their impact on system performance.In some embodiments, the FWM tones fall outside the bandwidth of areceiver of the data-bearing channels which may be adjacent to thecontrol channels. Since the control channel powers may be much higherthan the per-channel powers of the data-bearing channels, it isdesirable to dither the control channel lasers to suppress stimulatedBrillouin scattering (SBS). Note that the power of the control channelsis close to the power of the data-bearing channels during normaloperation. However, the power of the control channels is likely to behigher than the data-bearing channels when there are no dummy channelsor under fault conditions where channels have been dropped.

A control scheme for adjusting the power of the control channels totimely react to a 100 microsecond transient can be realized usingoptical filters that are the same width as the bins. This embodiment canprovide a fast total power measurement and feedback to control the powerof the control channels in each bin. The power can be controlled bychanging the current through the control channel lasers or bycontrolling a set of fast optical attenuators.

Referring again to FIG. 1, a coupling mechanism 25 couples the controlchannels to the fiber and receives optical feedback necessary to controlthe power of the control channels. The coupling mechanism may be asimple wavelength-independent coupler with an appropriately chosen tapratio, for example (90% express channels)/(10% control channels). Asecond coupling mechanism taps off a portion of the light which includeslight from the control channels so that the total power per frequencysub-band can be measured. This second coupler may have an appropriatelyselected tap ratio such as, for example, 95%/5%.

In an optical mesh deployment, there will be other line systemstransmitting to and from other locations which may have their own set ofcontrol channels. An advantage of the coupling method described hereinis that the WSS is able to extinguish control channels from othersources before adding new control channels from a local source. Intypical terrestrial applications, optical amplifiers are operated ingain control, whereas in typical submarine links, the amplifiers areoperated in total output power control. When total output power controlis used, a primary function of the control channels is to control theper-channel power of the dummy/data-bearing channels by saturating thetotal output power of the amplifiers in the line.

In some embodiments, the data-bearing channels are centered at regularfrequency intervals. Each of a pair of signals forming a control channelare separated by one half of the frequency interval that separatesadjacent data-bearing signals. The separation of the signals of acontrol channel signal pair may be selected to reduce four-wave mixingproducts on adjacent data-bearing channels. The signals forming the pairmay have an average frequency that is at a midpoint of a frequencyinterval. Further, the separation of the pair of signals forming acontrol channel may be chosen to be greater than a bandwidth of areceiver of an adjacent data bearing channel.

FIG. 4 is a block diagram of an exemplary channel controller 70 forcontrolling the power of a plurality of control channels. A plurality oflow line width lasers 50 provide each of a pair of signals for a controlchannel. The signals are combined in a polarization beam splitter 52 toproduce a control channel with two orthogonally polarized beams ofnarrow width. Two of the control channels are combined in a 3 dBcombiner 54 to produce two control channels which can be controlledtogether within one frequency sub-band. Each control channel signal pairis fed to a fast Variable Optical Attenuator (VOA), 56 where the gain ofthe signal pair is adjusted based on power measurements of the controlchannels. The control channels are combined by a wavelength divisionmultiplexer 58 which outputs the control channels to be input to a linesystem control channel input, coupling at least one control channel toeach of a plurality of frequency sub-bands.

A line system monitor output couples the signals on the line system to awavelength division demultiplexer 60 which separates the controlchannels along with any other channels within the respective frequencysub-band. The power of each control channel is detected by atrans-impedance amplifier (TIA) 62 and is coupled to ananalog-to-digital converter 64. The detected signals are input to adigital control system 66 that implements processes to generate anattenuator control signal that is based on the power of a detectedsignal. The processes for generating an attenuator control signal mayinclude a digital filter or recursive algorithm. The attenuator controlsignals are fed to digital-to-analog converters (DAC) 68. The DACsconvert the attenuator control signals to analog signals that are usedto control attenuation of the variable optical attenuators 56. Note thatin some embodiments, the digital control system 66 may be replaced byanalog circuitry that performs the same function.

Thus one embodiment is an apparatus for power control of signals carriedby an optical fiber. The apparatus includes a plurality of lasers 50 toproduce a plurality of control channels. Each control channel includes apair of signals at separate frequencies. There is at least one controlchannel in each of a plurality of frequency sub-bands (as shown in FIG.2). A plurality of attenuators 56 control attenuation of each controlchannel based on a plurality of feedback attenuation control signals. Aplurality of detectors 62 detect power in each of a plurality of opticalsignals received in the frequency sub-bands. A digital processor 66determines the plurality of feedback attenuation control signals basedon the detected power. The feedback attenuation control signals arecoupled to the plurality of attenuators 56 to control attenuation ofeach control channel.

Thus, one embodiment is an optical control channel signal generatorhaving a plurality of lasers to produce a plurality of signal pairs.Each signal pair forms a control channel in a separate one of aplurality of frequency sub-bands. The signals of a signal pair forming acontrol channel are separated by a first frequency interval. A combinercombines a first signal and a second signal of a signal pair so that apolarization of the first signal is orthogonal to a polarization of thesecond signal. The combiner may include a polarization beam combiner.The optical control channel signal generator has a plurality ofattenuators to attenuate a signal pair based on a measured optical powerof a sub-band. The plurality of attenuators control the amplitudes ofthe signal pairs of the control channels. For example, the attenuatorsmay control the amplitudes of the lasers producing the signals of thesignal pair or may control the attenuation of a plurality of variableoptical attenuators applied to the control channel signals. Adjustmentof a control channel attenuator may occur within 1 to 100 microsecondsof a change in optical power of a sub-band. Thus, compensation due to abreak in the fiber or another fault that affects the power profile ofthe fiber may occur in less than 100 microseconds.

In some embodiments, a set of dummy channels are provided to substitutefor data-bearing channels. The dummy channels have an optical power soas to maintain a substantially constant power profile on the opticalfiber. A dummy channel may contain filtered optical noise fromspontaneous emission from an optical amplifier.

FIG. 5 is a flow chart of an exemplary process for adjusting the powerof a control channel. In a first step, the total optical power of allchannels, data-bearing, dummy and control, for a sub-band is detected(step S102). The detected optical power is compared to a target opticalpower for the sub-band (step S104). Based on the comparison, adetermination of whether the optical power of the control channel mustbe adjusted is made (step S106). If the optical power of the controlchannel should be adjusted, the adjustment is made to match the opticalpower to the target optical power (step S108). The process thencontinues at step S102. If the optical power of the control channelsdoes not need adjustment, the process then continues at step 5102,without adjusting the optical power of the control channel.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed herein above. In addition, unless mention was made above tothe contrary, it should be noted that all of the accompanying drawingsare not to scale. A variety of modifications and variations are possiblein light of the above teachings without departing from the scope andspirit of the invention, which is limited only by the following claims.

What is claimed is:
 1. A method of loading a fiber optic transportsystem, the method comprising: adding one or more control channels whichare non-data-bearing to an optical fiber having one or more channelswhich are data-bearing, wherein each of the control channels comprises apair of signals that are cross-polarized and each of the pair of signalsis at a different frequency from one another; measuring optical power onthe optical fiber; and adjusting optical power of the one or morecontrol channels based on the measured optical power such that, for eachcontrol channel, the respective pair of signals at the differentfrequencies have their optical power controlled together.
 2. The methodof claim 1, wherein the adjusting optical power of a control channeladjusts power of each of the pair of signals for the control channel. 3.The method of claim 1, wherein the different frequencies between thepair of signals is selected to avoid a single polarization state of thecontrol channel.
 4. The method of claim 1, wherein the differentfrequencies between the pair of signals is selected so that four wavemixing tones of the one or more control channels fall on boundaries ofthe one or more channels.
 5. The method of claim 1, wherein thedifferent frequencies between the pair of signals is selected to begreat enough to avoid a single polarization state of the control channeland small enough to reduce non-linear interactions between the pair ofsignals of a control channel and signals of the one or more channelsnearby the control channel.
 6. The method of claim 1, wherein thedifferent frequencies between the pair of signals is 25 GHz.
 7. Themethod of claim 1, wherein lasers forming each of the pair of signals ofthe one or more control channels are dithered to suppress stimulatedBrillouin scattering (SBS).
 8. The method of claim 1, wherein the pairof signals forming a control channel are offset from spacing of the oneor more channels.
 9. The method of claim 1, wherein the differentfrequencies between the pair of signals is selected so that a differencebetween the different frequencies is greater than a bandwidth of areceiver of the one or more channels.
 10. The method of claim 1, whereinthe different frequencies between the pair of signals is selected toreduce four-wave mixing products on adjacent data-bearing channels. 11.A system for loading a fiber optic transport system, the systemcomprising: a plurality of optical sources configured to add one or morecontrol channels which are non-data-bearing to an optical fiber havingone or more channels which are data-bearing, wherein each of the controlchannels comprises a pair of signals that are cross-polarized and eachof the pair of signals is at a different frequency from one another; anoptical detector configured to measure optical power on the opticalfiber; and an optical power control unit configured to adjust opticalpower of the one or more control channels based on the measured opticalpower such that, for each control channel, the respective pair ofsignals at the different frequencies have their optical power controlledtogether.
 12. The system of claim 11, wherein the different frequenciesbetween the pair of signals is selected to avoid a single polarizationstate of the control channel.
 13. The system of claim 11, wherein thedifferent frequencies between the pair of signals is selected so thatfour wave mixing tones of the one or more control channels fall onboundaries of the one or more channels.
 14. The system of claim 1,wherein the different frequencies between the pair of signals isselected to be great enough to avoid a single polarization state of thecontrol channel and small enough to reduce non-linear interactionsbetween the pair of signals of a control channel and signals of the oneor more channels nearby the control channel.
 15. The system of claim 11,wherein the different frequencies between the pair of signals is 25 GHz.16. The system of claim 11, wherein the optical sources forming each ofthe pair of signals of the one or more control channels are dithered tosuppress stimulated Brillouin scattering (SBS).
 17. The system of claim1, wherein the pair of signals forming a control channel are offset fromspacing of the one or more channels.
 18. The system of claim 11, whereinthe different frequencies between the pair of signals is selected sothat a difference between the different frequencies is greater than abandwidth of a receiver of the one or more channels.
 19. The system ofclaim 11, wherein the different frequencies between the pair of signalsis selected to reduce four-wave mixing products on adjacent data-bearingchannels.
 20. A optical power control unit for control of loading in afiber optic transport system, the optical power control unit comprising:a connection to a plurality of optical sources configured to add one ormore control channels which are non-data-bearing to an optical fiverhaving one or more channels which are data-bearing, wherein each of thecontrol channels comprises a pair of signals that are cross-polarizedand each of the pair of signals is at a different frequency from oneanother; a connection to an optical detector; and a processor configuredto obtain measure optical power on the optical fiber from the opticaldetector; and cause adjustment of optical power of the plurality ofoptical sources based on the measure optical power such that, for eachcontrol channel, the respective pair of signals at the differentfrequencies have their optical power controlled together.